Comparison of the Dynamic and Static Shear Strengths of Cohesionless, Cohesive and Combined Soils

摘要

A direct shear device capable of applying maximum shear stresses to soil specimens in a period of time ranging from 1 millisecond to 20 minutes has been utilized to test a wide variety of soils. The cohesionless materials, an Ottawa sand in the loose and dense condition, a powdered Nevada silt and a dry powder clay, did not exhibit any increase in maximum shear resistance due to an impact type dynamic shear force application as compared to a static force application. An increase of apparent friction angle from 45 degrees to approximately 60 degrees due to inertial confinement was observed in a dense Ottawa sand. Cohesive materials, which included undisturbed and remolded clays and combined soils (mixtures of sand and clay), demonstrated an increase in maximum shear resistance under impact loads described solely by the apparent cohesion intercept of the failure envelope. The friction angle was essentially insensitive to test duration. The ratio of the apparent cohesion for a failure envelope involving failure times of 5 milliseconds to the corresponding intercept for failure times of nearly 1 minute was approximately 2. This ratio appeared to be relatively insensitive to moisture content, dry density, grain size and soil structure (flocculated or dispersed) for degrees of saturation in excess of 85%. The apparent cohesion ratio appeared to decrease on the dry side of optimum for compacted soils. (Author)